Methane clathrates are icy deposits on the sea-floor which contain large amounts of methane (a powerful greenhouse gas). One of the big concerns with climate change is that rising sea temperatures could lead to a release of some of this methane, causing further warming and positive feedback. This process is implicated in two mass-extinction events - the Paleocene–Eocene Thermal Maximum 56 million years ago, and the Permian–Triassic extinction event 251 million years ago which wiped out about 90% of all life on earth.
The bad news is that it seems to be happening - clathrate beds on the arctic seafloor have started to thaw:
Russian polar scientists have strong evidence that the first stages of melting are underway. They've studied largest shelf sea in the world, off the coast of Siberia, where the Asian continental shelf stretches across an underwater area six times the size of Germany, before falling off gently into the Arctic Ocean. The scientists are presenting their data from this remote, thinly-investigated region at the annual conference of the European Geosciences Union this week in Vienna.This is seriously bad news, for us and for the planet as a whole. And it suggests we may be close to a tipping point in the climate - that is if we're not past it already.[...]
The permafrost has grown porous, says Shakhova, and already the shelf sea has become "a source of methane passing into the atmosphere." The Russian scientists have estimated what might happen when this Siberian permafrost-seal thaws completely and all the stored gas escapes. They believe the methane content of the planet's atmosphere would increase twelvefold. "The result would be catastrophic global warming," say the scientists. The greenhouse-gas potential of methane is 20 times that of carbon dioxide, as measured by the effects of a single molecule.